Volcanic Components Research Articles

Gas geochemistry of the Cordón Caulle geothermal system, Southern Chile

The Cordón Caulle geothermal system is located in a NW-trending volcano-tectonic depression of the Southern Andean Volcanic Zone of Chile. Outflows of low chloride water were previously interpreted as the surface expression of a shallow steam-heated aquifer, with subsurface temperatures of 150–170 °C. Gas data from fumaroles and hot springs have been used to assess the nature and temperature of the deeper, underlying geothermal reservoir. Fumaroles at the northeastern border of Cordón Caulle have 3He/ 4He ratios typical of subduction margins (6–7 R A) and N 2/Ar ratios of about 40, indicating deep convection of air-saturated groundwater. Fumaroles at the southwestern border have N 2/Ar ratios >300, suggesting the presence of a deep volcanic component. Gas ratios of fumarole discharges yield equilibration temperatures >300 °C, whereas those of hot spring waters suggest temperatures of about 160 °C. Based on these data, and comparisons with well documented liquid and vapor-dominated geothermal systems, a model is proposed of a boiling liquid-dominated geothermal system overlain by a secondary steam-heated aquifer.

Late Pleistocene to Holocene temporal succession and magnitudes of highly-explosive volcanic eruptions in west-central Nicaragua

The stratigraphic succession of widespread tephra layers in west-central Nicaragua was emplaced by highly explosive eruptions from mainly three volcanoes: the Chiltepe volcanic complex and the Masaya and Apoyo calderas. Stratigraphic correlations are based on distinct compositions of tephras. The total tephras combine to a total on-shore volume of about 37 km 3 produced during the last ∼ 60 ka. The total erupted magma mass, including also distal volumes, of 184 Gt (DRE) distributes to 84% into 9 dacitic to rhyolitic eruptions and to 16% into 4 basaltic to basaltic–andesitic eruptions. The widely dispersed tephra sheets have up to five times the mass of their parental volcanic edifices and thus represent a significant albeit less obvious component of the arc volcanism. Eruption magnitudes ( M = log 10( m) − 7 with m the mass in kg), range from M = 4.1 to M = 6.3. Most of the eruptions were dominantly plinian, with eruption columns reaching variably high into the stratosphere, but minor phreatomagmatic phases were also involved. Two phreatomagmatic eruptions, one dacitic and one basaltic–andesitic, produced mostly pyroclastic surges but also fallout from high eruption columns. Comparison of fallout tephra dispersal patterns with present-day, seasonally changing height-dependant wind directions suggests that 8 eruptions occurred during the rainy season while 5 took place during the dry season. The tephra succession documents two major phases of erosion. The first phase, > 17 ka ago, appears to be related to tectonic activity whereas the second phase may have been caused by wet climatic conditions between 2 to 6 ka ago. The Apoyo caldera had two large plinian, caldera-forming eruptions in rapid succession about 24 ka ago and should be considered a silicic volcano with long repose times. Three highly explosive basaltic eruptions were generated at the Masaya Caldera within the last 6 ka. Since then frequent but small eruptions and lava effusion were largely limited to the caldera interior. The dacitic Chiltepe volcanic complex experienced six plinian eruptions during the last 17 ka and seems to be an accelerating system in which eruption magnitude increased while the degree of differentiation of erupted magma decreased at the same time. We speculate that the Chiltepe system might produce the next large-magnitude silicic eruption in west-central Nicaragua.

Geochemical characterization of volcanic ashes from recent events in Northern Patagonia Andean Range by INAA

Volcanic ashes erupted by recent magmatic events were collected from one sedi mentary sequence of lake Nahuel Huapi (Patagonia, Argentina) and then analyzed by Instrumental Neutron Activation Analysis. Primary volcanic components, including white and brownish pumice fragments, scoriae and glass shards, as well as bulk sediments from four tephra layers identified in the sedimentary sequence, were characterized by their major and trace element contents. The two tephra layers being in the upper section of the sequence can be linked to the eruptions of Cordon Caulle fissure (Puyehue -Cord on Caulle region) occurred in 1921 –1922 and 1960, according to core dating. The third tephra layer, which is clearly associated to an event occurred in the 1890 –1900 decade, has different compositional patterns showing a different source from those upper t ephras. Whereas, the fourth, which could be associated to an event occurred at the end of the 18 th century, shows the same composition patterns as those registered for the Cordon Caulle fissure.

Eddy covariance measurements of hydrothermal heat flux at Solfatara volcano, Italy

The first measurements of volcanic/hydrothermal water vapor and heat flux using eddy covariance (EC) were made at Solfatara crater, Italy, June 8–25, 2001. Deployment at six different locations within the crater allowed areas of focused gas venting to be variably included in the measured flux. Turbulent (EC) fluxes of water vapor varied between 680 and 11 200 g H 2O m − 2 d − 1 . Heat fluxes varied diurnally with the solar input, and the volcanic component of sensible heat ranged from ∼25 to 238 W m − 2 . The highest measurements of both sensible and latent heat flux were made downwind of hot soil regions and degassing pools and during mid-day. The ratio of average volcanic heat (both latent and sensible) to CO 2 flux resulted in an equivalent H 2O/CO 2 flux ratio of 2.2 by weight, which reflects the deep source H 2O/CO 2 gas ratio. The amount latent heat flux/evaporation was determined to be consistent both with what would be expected from the magnitude of CO 2 fluxes and the fumarolic H 2O/CO 2 ratio, as well as with observed surface temperatures and wind speeds given a moist soil. This suggests that the water vapor that condenses in the shallow subsurface is remobilized at the soil–atmosphere interface through variable evaporation dependent on the deep heat flux and surface temperature. The results suggest that EC provides a quick and easy method to monitor average H 2O/CO 2 ratios continuously in volcanic regions, providing another important tool for volcanic hazards monitoring.

Youngest volcanism about 1 million years ago at Kahoolawe Island, Hawaii

Young volcanic deposits in Kahoolawe Island, cutting up through the caldera-filling lava, colluvium and talus in the west wall of Kanapou Bay, had long been stratigraphically considered the rejuvenated-stage products. New K–Ar ages, combined with magnetic polarity data, show that young volcanism was at about 0.98–1.04 Ma and indicate no substantial quiescence between the filling of the caldera and the young volcanism. This result, and the tholeiitic characteristics of the young deposits, suggest they are a component of late shield-stage volcanism.

Chemistry of rainwaters in the south Pacific area of Russia

On the south-eastern edge of Russia, the chemical composition of rainwater is controlled by sea salts, terrestrial material, as well as volcanic (Kuril islands volcanic area) and anthropogenic emissions, mostly in the southern part of the area. The predominant major ions of the Primorye, Sakhalin and the Kuril Islands rainwaters were respectively HCO 3 −–SO 4 2−, Ca–Na, and of Cl–Na. Concentration of trace elements changes within 1–2 orders of magnitude but some difference in the distribution of the elements between continental and island rainwater is found. The concentration of the chemical elements in the particulate fraction varies from < 10% to 90% of the total concentration (dissolved + particulate) with the following distribution: Tl, Na, Ca, Sr, Zn, Cd (< 10%)–Be, Th, Bi, Rb, U, K, Sc (10–20%)–Cu, Mn, Mg, Mo, Se, Ba, Ni, As, Ag, Cs, Co, Y, Ga, V (20–50%)–Sb, Pb, Ge, Cr, Fe, Al (50–90%). The concentration of elements of the particulate fraction of the rainwater usually is significantly different from concentrations in the crust, including both higher and lower concentrations. The terrestrial contribution to dissolved elements was evaluated and follows the decreasing order: Fe > K, Mg, Ca > Ba, Sr > Na (65–1%). Close order was found for total (dissolved and solid) concentrations. Sea salt contribution to dissolved element concentration in the rainwater decrease in the following order: Cl, Mg > K, SO 4 > Ca > HCO 3 −, Ba, Fe (78–0.1%). Calculation of anthropogenic and volcanic inputs for two ions (Cl − and SO 4 2−) shows that anthropogenic inputs for the Vladivostok and Yuzno-Sakhalinsk cities can be evaluated as 15–20% of Cl − and up to 80–90% of SO 4 2−. Volcanic components in the Kuril Islands, where anthropogenic inputs are absent, can reach up to 76% of SO 4 2− and 36% of Cl −.

The crustal structure and regional development of the Irish Atlantic margin region

During the past decade, a suite of wide-angle seismic reflection/refraction profiles has been shot in the Porcupine, Rockall and Hatton basins, as well as across the Hatton continental margin. Integration of the wide-angle seismic data with normal-incidence reflection profiles and with gravity and magnetic data reveals a clear picture of the regional crustal and upper mantle structure, and of the large-scale sedimentary geometry of the Late Palaeozoic–Cenozoic basins in the area. The region contains a set of large sedimentary basins resting upon variably thinned continental crust. The crust beneath the basin-bounding structural highs is typically around 25km thick, with the crust beneath the basins being as thin as 6km. The Porcupine Basin contains up to 10 km of sediments. Up to 7km of sedimentary strata are indicated in the Rockall Basin, with 4km of sediment in the Hatton Basin. Several seismically distinct layers, with P-wave velocities of 1.8–5.3 kms −1 , were identified above the basement in the sedimentary basins. A significant variability in thickness of the sedimentary layers is seen on the wide-angle data. These reflect the presence of a number of older Mesozoic rift sedimentary basins. The wide-angle data reveal marked topographic irregularity at the base of the sedimentary succession in the Rockall Basin, interpreted as the result of extensive multi-phase Mesozoic rifting. The Moho beneath the Rockall Basin shows a clear asymmetry, with a steeper gradient along the eastern margin of the basin. The continent–ocean boundary west of the Hatton High has a significant underplated and volcanic component, while that at the southern margin of the Rockall Basin shows evidence of a greater transpressive component. The nature of the lower crust appears to exert a controlling influence on the location of crustal separation. In areas where thinning is accommodated by upper and middle crustal attenuation, the strength of the upper crust (and underlying brittle mantle) has served to preserve the integrity of the continental crust and inhibit seafloor spreading. However, where the lower crust is thinned significantly, as beneath the Hatton continental margin, crustal separation has taken place even though the upper and middle crust remains relatively thick.

Late Cenozoic evolution of the Nankai trench-slope system: evidence from sand petrography and clay mineralogy

Abstract Submarine slope systems in subduction zones evolve in response to a combination of tectonic and sedimentary forcing. It can be difficult to determine how and when tectonic forcing affects sedimentation, especially when investigating ancient rock successions, but one of the more reliable indicators is a change in sediment composition. During Leg 190 of the Ocean Drilling Program, sandy turbidites were recovered from a Quaternary trench wedge (Nankai Trough), a Pliocene-Pleistocene slope basin, the underlying Pliocene-Miocene accretionary prism, and a Miocene turbidite facies in the Shikoku Basin. Differences in detrital provenance between the sand and clay-sized fractions indicate that turbidity currents did not follow pathways of suspended-sediment transport during the past 10 Ma. During the middle and late Miocene, the sand probably was eroded from a newly exposed accretionary complex (Shimanto Belt). In contrast, high contents of detrital smectite in Miocene mudstones (&gt;50 wt% of the &lt;2 μm size fraction, relative to illite, chlorite + kaolinite, and quartz) point to a strong volcanic component of suspended-sediment input (Izu-Bonin island arc). The sand in accreted Pliocene turbidites was also eroded from the Shimanto Belt and transported by transverse flow down the insular slope. The trench-wedge facies then switched to axial flow during the Quaternary, when the sand supply tapped a mixed volcanic-metasedimentary provenance in the rapidly uplifted Izu-Honshu collision zone. Progressive depletion of smectite during the Pliocene and Pleistocene (&lt;20 wt%) points to increased movement of illite- and chlorite-rich clay toward the east and NE from sources on Kyushu and Shikoku. That shift in mud composition coincides with intensification of the North Pacific western boundary current (Kuroshio Current) at approximately 3 Ma. Overall, the depositional system in the Nankai Trough and Shikoku Basin shifted its sand sources because of regional tectonics, whereas the suspended-sediment budget was modulated by hemispheric changes in ocean-water circulation.

Microstructural features of albite porphyroblasts as indicators of sequential Barrovian metamorphic mineral growth in the Caledonides of the SW Scottish Highlands

AbstractInclusion – porphyroblast and porphyroblast – porphyroblast relationships show that abundant albite in mica schists in the Caledonides of the SW Scottish Highlands are part of the Barrovian metamorphic assemblage. Growth early in the D2 deformational phase of porphyroblast cores followed the growth of Mn‐rich garnet but preceded the growth of porphyroblasts of the index mineral almandine. Two sets of inclusion trails in the albite correspond to the regionally expressed S1 and S2. Straight trails of muscovite, chlorite, quartz, epidote and the earliest growth of biotite make up S1. Crenulated trails express deformation of S1 early in D2 with muscovite, chlorite, biotite, quartz, epidote and the Mn‐rich garnet associated with the development of S2 crenulation cleavage. The geometries of these trails uniquely record early stages of D2 deformational history. An 0−3 growth is related to the temporal coincidence of the formation of S1–S2 crenulation cleavage hinges as favourable sites for nucleation and the release of large amounts of water from prograde reactions during tectonothermal reconstitution of first cycle immature sediments with a volcanic component. The main characteristics of the regionally expressed D2 schistosity were developed during the major grain coarsening that followed both albite and almandine porphyroblast growth. Essentially inclusion‐free An 4−19 rims grew on the inclusion‐containing cores in the almandine zone in the later stages of schistosity growth and unoriented porphyroblasts of muscovite, biotite and chlorite indicate that mineral growth extended from the later stages of D2 to post‐D2. Previous interpretations of the albite porphyroblast growth having been during D4 to post‐D4 contemporaneous with retrogression are inconsistent with the microstructural evidence.

Nd and Pb isotope signatures of the clay‐size fraction of Labrador Sea sediments during the Holocene: Implications for the inception of the modern deep circulation pattern

Nd and Pb isotopes were measured on the fine fraction of one sediment core drilled off southern Greenland. This work aims to reconstruct the evolution of deep circulation patterns in the North Atlantic during the Holocene on the basis of sediment supply variations. For the last 12 kyr, three sources have contributed to the sediment mixture: the North American Shield, the Pan‐African and Variscan crusts, and the Mid‐Atlantic Ridge. Clay isotope signatures indicate two mixtures of sediment sources. The first mixture (12.2–6.5 ka) is composed of material derived from the North American shield and from a “young” crustal source. From 6.5 ka onward the mixture is characterized by a young crustal component and by a volcanic component characteristic of the Mid‐Atlantic Ridge. Since the significant decrease in proximal deglacial supplies, the evolution of the relative contributions of the sediment sources suggests major changes in the relative contributions of the deep water masses carried by the Western Boundary Undercurrent over the past 8.4 kyr. The progressive intensification of the Western Boundary Undercurrent was initially associated mainly with the transport of the Northeast Atlantic Deep Water mass until 6.5 ka and with the Denmark Strait Overflow Water thereafter. The establishment of the modern circulation at 3 ka suggests a reduced influence of the Denmark Strait Overflow Water, synchronous with the full appearance of the Labrador Seawater mass. Our isotopic data set emphasizes several changes in the relative contribution of the two major components of North Atlantic Deep Water throughout the Holocene.

Oxygen isotope composition of natural waters in the Mt Etna area

Oxygen isotopes of both rainwater and groundwater samples from Mt Etna were used to obtain information on recharge areas, flow paths, and the origin of wet air masses. Oxygen isotope composition was determined in rainwater samples collected for a period of 3 years (October 1997–October 2000), in 11 rain-gauges distributed along the flanks of Mt Etna from sea level to 2900 m of altitude. Values ranged from −13.8 to+1.9‰, the lowest values being measured at higher altitudes and/or during cooler periods. For rain-gauges located from sea level up to 1000 m altitude, volume weighted values defined an isotopic gradient of −2.7‰/km, which is in the range observed in the Mediterranean area. Higher-altitude gauges yielded a much lower gradient (−0.6‰/km), probably due to the fact that vapour condensing at higher altitudes was mixed with an 18O-rich volcanic component deriving from the huge vapour output of the summit craters. The oxygen isotope composition of about 210 groundwater samples collected all around the volcano ranged from −9.3 to −5.0‰. The higher values measured on the eastern flank indicated that recharge occurs at lower altitudes on this flank. The low variability (0.30–0.65‰) of the monthly values gathered from 14 groundwater sampling points over a period of 2 years indicates that the groundwater system is isotopically well mixed. Some long-term trends may be explained by variations in annual recharge, due to the prevalent isotope composition of wet air masses.

Facies architecture of the Lewis Ponds carbonate and volcanic‐hosted massive sulfide deposits, central western New South Wales

The Lewis Ponds carbonate and volcanic‐hosted Zn–Pb–Cu–Ag–Au–rich massive sulfide deposits are located near the western margin of the Hill End Trough in the eastern Lachlan Fold Belt of New South Wales. Two stratabound massive sulfide zones, Main and Toms' occur in a tightly folded Upper Silurian succession of marine felsic volcanic and sedimentary rocks. Siltstone, mudstone and quartz crystal‐rich sandstone overlie thick, tabular beds of poorly sorted mixed‐provenance breccia, limestone‐clast breccia and pebbly granular sandstone. The sedimentary rocks unconformably overlie a succession of strongly foliated, quartz‐plagioclase‐phyric dacite. Hydrothermal alteration and deformation have obscured primary textures and contact relationships in the footwall volcanic succession. However, three lithofacies associations were distinguished using phenocryst abundance, overall texture and immobile element compositions. Thick (up to 100 m) conformable or fault‐bound units of poorly sorted limestone‐clast breccia, megabreccia and massive limestone occur throughout the Lewis Ponds host sequence. Clasts vary in size from small pebbles to 10 m–long boulders and 90 m–thick lenses. Where primary textures are preserved, the bioclastic limestone contains crinoid fragments, brachiopods, bivalves, solitary corals, volcanic quartz and feldspar crystals and rare Silurian conodonts. Although originally deposited in shallower water, the allochthonous limestone occurs in a host sequence of predominantly siltstone and mudstone. The mixed‐provenance breccia, limestone‐clast breccia, pebbly granular sandstone and quartz crystal‐rich sandstone were deposited on the flanks of a high‐level intrusive dacite centre in a moderately deep, below wave‐base slope environment. Volcanic and sedimentary components originated from multiple source areas at the basin margin and in the adjacent hinterland. The Lewis Ponds host sequence records a transition from coarse‐grained polymictic volcaniclastic breccia and sandstone, upwards into mudstone, siltstone and minor volcanogenic sandstone, indicating deposition in a deepening, more restricted environment as volcanism waned and the basin progressively subsided.

A new lithostratigraphic framework for the Anti-Atlas Orogen, Morocco

A new lithostratigraphic nomenclatural framework is proposed for the Anti-Atlas Orogen of Morocco, to replace the previous chronostratigraphic scheme. All the lithostratigraphic units of the Anti-Atlas Orogen are well represented in the Sirwa, Kerdous and Sarhro inliers and examples from these areas are given prominence, though correlates in other areas are given, where known. The oldest, Palaeoproterozoic, rocks of the cratonic basement form the northern part of the West African Craton and can be subdivided into a series of Complexes (made up of constituent Groups, Suites etc.), depending on geographical outcrop (e.g. the Zenaga and Kerdous Complexes in the Sirwa and Kerdous inliers respectively). The Neoproterozoic rocks deposited on this basement are termed the Anti-Atlas Supergroup, within which five groups of volcano–sedimentary units ( Taghdout, Jbel Lkst, Sarhro, Iriri, and Bou Azzer Groups) and various intrusive igneous rocks are recognised (e.g. Ifzwane and Toudma Suites). These units are related to the earliest passive margin, oceanic and island-arc development phases of the orogen between ∼800 and 660 Ma. The earliest Pan-African deformation (at ∼660 Ma) resulted from closure of the ocean basin, SW-directed thrusting and accretion of the island-arc remnants. Continued prolonged convergence gave rise to a sinistral transpressional regime in which late syn- to post-tectonic continental volcanic and clastic sequences of the Ouarzazate Supergroup were deposited. The first rocks of this new tectonic regime were deposited in narrow, tectonically active, strike-slip pull-apart rift basins ( Bou Salda, Mgouna, Tafrawt and Anzi Groups) associated with intrusion of high-K calc-alkaline granitoid batholiths (e.g. Assarag, Bardouz Suites) and the juxtaposition of ophiolitic remnants in major strike-slip shear belts. These were followed by the deposition of the much more areally extensive continental volcano–sedimentary molasse, the volcanic components of which were erupted from several interfingering volcanic centres, often associated with caldera complexes and sub-volcanic alkaline granites ( Toufhgrane, Tanghourt and Guellaba Suites). The proposed lithostratigraphic scheme encompasses all the major rock units identified in the literature from the orogen and is flexible so that new, lower-ranking, units can be placed at the appropriate stratigraphic level in future. This framework is supported by new high-precision U–Pb zircon dates that have recently become available.

Effects of nanoscale pores in soils on carbon evolution under extremely dry conditions

Soil carbon evolution under the lowest moisture conditions varies considerably among experimental systems/techniques, leading to discrepancies in the estimations from carbon dynamics models under low moisture conditions. We focused our study on clarifying the regulating factors of soil carbon evolution under the lowest moisture conditions by conducting laboratory experiments under precisely controlled conditions. Nanoscale porosity and surface properties of these soils were determined to analyze the role of residual water in the carbon evolution processes in dry soils. Laboratory incubation showed that the carbon evolution from a microporous (D < 2 n m) volcanic soil proceeded even at -100 U kg-1 water potential (INT) in contrast to the carbon evolution from a phyllosilicate alluvial soil. Pore-size estimation and 1H-NMR spectroscopy showed that the carbon evolution at -100 U kg-1 WP proceeded through the utilization of nanopore-water in soils. Batch sorption experiment suggested that the surface affinity of the soils to dissolved organic matter (DOM) had enhanced carbon evolution by attracting DOM into hydrophilic spheres of the soil at -100 U kg-1 MT. Solid-state IIGNMR of organic matter samples (incubated in the absence of soils) suggested that the chemical alteration of the samples was significant for aliphatic components, while the alteration was not observed in the samples incubated at -100 U kg-1 WP. This fact also indicated the contribution of nanoscale pores in the volcanic components to carbon evolution. Application of the experimental results to several biogeochemical models revealed that both volumetric water content and MT are required to estimate carbon evolution under low moisture conditions. A micro habitat model showed that the carbon evolution at -100 U kg-1 WP could be attributed to extracellular enzymatic processes or other abiotic processes rather than to the activities of living microorganisms.

Northwest Africa 773: lunar mare breccia with a shallow-formed olivine-cumulate component, inferred very-low-Ti (VLT) heritage, and a KREEP connection

Lunar meteorite Northwest Africa 773 (herein referred to as NWA773) is a breccia composed predominantly of mafic volcanic components, including a prominent igneous clast lithology. The clast lithology is an olivine-gabbro cumulate, which, on the basis of mineral and bulk compositions, is a hypabyssal igneous rock related compositionally to volcanic components in the meteorite. The olivine-gabbro lithology exhibits cumulus textures and, in our largest section of it, includes some 48% olivine (Fo 64 to Fo 70, average Fo 67), 27% pigeonite (En 60Fs 24Wo 16 to En 67Fs 27Wo 6), 11% augite (En 50Fs 17Wo 33 to En 47Fs 13Wo 40), 2% orthopyroxene (En 70Fs 26Wo 4), 11% plagioclase (An 80 to An 94), and trace barian K-feldspar, ilmenite, Cr-spinel, RE-merrillite, troilite, and Fe-Ni metal. The Mg/Fe ratios of the mafic silicates indicate equilibration of Fe and Mg; however, the silicates retain compositional variations in minor and trace elements that are consistent with intercumulus crystallization. Accessory mineralogy reflects crystallization of late-stage residual melt. Both lithologies (breccia and olivine cumulate) of the meteorite have very-low-Ti (VLT) major-element compositions, but with an unusual trace-element signature compared to most lunar VLT volcanic compositions, i.e., relative enrichment in light REE and large-ion-lithophile elements, and greater depletion in Eu than almost all other known lunar volcanic rocks. The calculated composition of the melt that was in equilibrium with pyroxene and plagioclase of the cumulate lithology exhibits a KREEP-like REE pattern, but at lower concentrations. Melt of a composition calculated to have been in equilibrium with the cumulate assemblage, plus excess olivine, yields a major-element composition that is similar to known green volcanic glasses. One volcanic glass type from Apollo 14 in particular, green glass B, type 1, has a very low Ti concentration and REE characteristics, including extremely low Eu concentration, that make it a candidate parent melt for the olivine-gabbro cumulate. We infer an origin for the parent melt of NWA773 volcanic components by assimilation of a trace-element-rich partial or residual melt by a magnesian, VLT magma deep in the lunar crust or in the mantle prior to transportation to the near-surface, accumulation of olivine and pyroxene in a shallow chamber, eruption onto a volcanic surface, and incorporation of components into local, predominantly volcanic regolith, prior to impact mixing of the volcanic terrain and related hypabyssal setting, and ejection from the surface of the Moon. Volcanic components such as these probably occur in the Oceanus Procellarum region near the site of origin of the green volcanic glasses found in the Apollo 14 regolith.

U-Pb (Zircon) ages of metavolcanic rocks from the Itaiacoca Group: tectonic implications

The main aim of this work is to present and discuss the U-Pb ages obtained for zircon grains from metavolcanic rocks of the Itaiacoca Group. The Itaiacoca Group is a metavolcano-sedimentary sequence, which occurs as a narrow belt between the Cunhaporanga granitic batholith to the northwest and the Itapirapua shear zone to the south and southwest, which separates the sequence from the Tres Corregos granite batholith and metasedimentary rocks of the Acungui Group. Geological studies of the southern part of the Itaiacoca belt led to the recognition of three units, represented (from base to top) by metawackes with an important volcanic component, metacarbonate, and metapelitic and metapsammitic rocks. The U-Pb geochronological analyses of zircon grains from two outcrops of metavolcanic rocks yield ages of 628 ± 18 Ma (SHRIMP) and 636 ± 30 Ma (conventional multi-grain analyses). These ages are quite close to the metamorphic event recorded in the Itaiacoca Group (628 - 610 Ma), suggesting a short interval between the formation of these rocks and closure of the basin. Furthermore, this volcanism is very close to the age of formation of the Tres Corregos (630 Ma) and Cunhaporanga (590 Ma) granitic batholiths, admitted as associated with a probable magmatic arc. Such an isotopic pattern characterizes a Neoproterozoic tectonic scenario involving volcanism, metamorphism and granitic plutonism, interpreted here as the final stages in the evolution of the Itaiacoca Basin.

Late Cenozoic volcanism in the western Woodlark Basin area, SW Pacific: the sources of marine volcanic ash layers based on their elemental and Sr–Nd isotope compositions

Tephra fallout layers and volcaniclastic deposits, derived from volcanic sources around and on the Papuan Peninsula, form a substantial part of the Woodlark Basin marine sedimentary succession. Sampling by the Ocean Drilling Program Leg 180 in the western Woodlark Basin provides the opportunity to document the distribution of the volcanically-derived components as well as to evaluate their chronology, chemistry, and isotope compositions in order to gain information on the volcanic sources and original magmatic systems. Glass shards selected from 57 volcanogenic layers within the sampled Pliocene–Pleistocene sedimentary sequence show predominantly rhyolitic compositions, with subordinate basaltic andesites, basaltic trachy-andesites, andesites, trachy-andesites, dacites, and phonolites. It was possible to correlate only a few of the volcanogenic layers between sites using geochemical and age information apparently because of the formation of strongly compartmentalised sedimentary realms on this actively rifting margin. In many cases it was possible to correlate Leg 180 volcanic components with their eruption source areas based on chemical and isotope compositions. Likely sources for a considerable number of the volcanogenic deposits are Moresby and Dawson Strait volcanoes (D'Entrecasteaux Islands region) for high-K calc-alkaline glasses. The Dawson Strait volcanoes appear to represent the source for five peralkaline tephra layers. One basaltic andesitic volcaniclastic layer shows affinities to basaltic andesites from the Woodlark spreading tip and Cheshire Seamount. For other layers, a clear identification of the sources proved impossible, although their isotope and chemical signatures suggest similarities to south-west Pacific subduction volcanism, e.g. New Britain and Tonga–Kermadec island arcs. Volcanic islands in the Trobriand Arc (for example, Woodlark Island Amphlett Islands and/or Egum Atoll) are probable sources for several volcaniclastic layers with ages between 1.5 to 3 Ma. The Lusancay Islands can be excluded as a source for the volcanogenic layers found during Leg 180. Generally, the volcanogenic layers indicate much calc-alkaline rhyolitic volcanism in eastern Papua since 3.8 Ma. Starting at 135 ka, however, peralkaline tephra layers appear. This geochemical change in source characteristics might reflect the onset of a change in geotectonic regime, from crustal subduction to spreading, affecting the D'Entrecasteaux Islands region. Initial 143Nd/144Nd ratios as low as 0.5121 and 0.5127 for two of the tephra layers are interpreted as indicating that D'Entrecasteaux Islands volcanism younger than 2.9 Ma occasionally interacted with the Late Archean basement, possibly reflecting the mobilisation of the deep continental crust during active rift propagation.

An experimental study of alteration of oceanic crust and terrigenous sediments at moderate temperatures (51 to 350°C): insights as to chemical processes in near-shore ridge-flank hydrothermal systems

Studies of hydrothermal circulation within partly buried basement on the eastern flank of the Juan de Fuca Ridge (JFR) have shown that ridge-flank geochemical fluxes are potentially important for the global budgets of some elements. There are major uncertainties in these flux calculations, however, because the composition of these basement fluids is strongly dependent on temperature and because they may be modified by interaction with the overlying terrigenous sediments, either by diffusive exchange with basement or during upwelling to the seafloor. To better understand the nature and temperature control of basalt-fluid and sediment-fluid reactions at the JFR flank, we have conducted laboratory experiments between 51 and 350°C and at 400 bars pressure. K, Rb, and Si are leached from basalt between 150 and 351°C, and Sr and U are taken up. The direction of exchange of Li and Ca with basalt varies with temperature. Li and Sr are removed from fluid at 150°C, but isotope studies show that there is simultaneous release of both elements from basalt, indicating that uptake is controlled by the formation of secondary minerals. Moreover, our experiments confirm that Sr isotope exchange with oceanic crust occurs at moderate temperature and is not confined to high-temperature axial hydrothermal systems. Our data and field data from the JDR flank indicate that uptake of U into basalt at moderate temperature could remove between 9.9 and 15 × 10 6 mol U yr −1 from the oceans. This is higher than a recent estimate based on measurements of U in altered ocean crust (5.7 ± 3.3 × 10 6 mol yr −1), which concords with arguments that the Δ element/heat ratios of JDR flank fluids are too large to be representative of average global flank fluids. K, Ca, Sr, Ba, Li, Si, and B are leached from terrigenous sediments between 51 and 350°C, and U is taken up. Cs and Rb are removed from the fluid below 100°C and leached from the sediment at higher temperature. Sr isotope data show that Sr is preferentially mobilised from volcanic components within terrigenous sediments, which may lead to an overestimation of the ridge-flank Sr isotope flux at the JDR if there is exchange of sediment pore fluids with basement.

The tropospheric sulphur cycle and the role of volcanic SO 2

Abstract A global three-dimensional chemistry-transport model has been applied to study the tropospheric sulphur cycle, and in particular the volcanic component. The model is in general agreement with previous studies of the global S budget. We find that volcanic emissions constitute 10% of the present-day global SO 2 source to the atmosphere, but form 26% of the SO 2 burden, and 14% of the sulphate aerosol burden. Two previous modelling studies suggested that the volcanic fraction of sulphate was 18% and 35%, from sources representing 7% and 14%, respectively, of the global total SO 2 emission. The results are dependent upon various assumptions about volcanic emissions (magnitude, geographical location, altitude), the global distribution of oxidants, and the physical processes of dry and wet deposition. Because of this dependence upon poorly constrained parameters, it is unclear which modelling study is closest to the truth.

Unroofing of the southern Front Range, Colorado: A view from the Denver Basin

Fission-track (FT) thermochronology of detrital apatite and zircon from synorogenic sedimentary rocks in the Kiowa Cored Well and the Castle Pines drillhole in the Denver Basin can be used to document a generally predictable unroofing sequence for the southern part of the Colorado Front Range. First, Phanerozoic sedimentary rocks that covered the Front Range in late Mesozoic time contributed recycled sedimentary and far-traveled volcanic zircon and apatite to the oldest sediments as Laramide deformation began. Next, two significant pulses of volcanic grains are recorded in both the detrital zircon and apatite FT age populations in the Kiowa Cored Well as volcanoes along the Colorado Mineral Belt dominated the landscape. The volcanic contribution is not as significant in the Castle Pines well, which is more proximal to the mountain front compared to the Kiowa Cored Well. Higher in the stratigraphic sequence, a mix of volcanic grains, recycled grains from the Phanerozoic cover, and Proterozoic basement grains is present, with the percentage of basement grains in the mixture increasing up section. Finally, in the youngest part of the sequence, metamict zircon grains from the basement and apatite grains, derived from the Proterozoic basement with 50 to 70 Ma apatite fission-track cooling ages derived from below the base of the apatite partial annealing zone in the Front Range, dominate the age populations. Complexities in the form of pulses of volcanic or basement components are superimposed on the simple pattern. This study demonstrates the power of analyzing both apatite and zircon when examining detrital grains derived from a basement-cored uplift where basement resided at shallow levels of the crust (